ERT is described as using low frequency electric current which is injected into the ground from electrodes on the surface or in boreholes. In a typical arrangement, the electrodes may be located on a well casing disposed within a borehole. The electrical potential induced by this current is measured using a second pair of electrodes and an automated system is used to collect data for different combinations of transmitting (current) and receiving (potential) electrodes. That is, in ERT, each measurement uses four electrodes; one pair of electrodes serves as the current source and sink and a second pair measures the potential difference between two points. For a system with N electrodes there are approximately N4 different configurations referred to as arrays.
The electrodes are connected to one or more data acquisition systems, typically using multi-conductor, insulated cables. Each electrode is connected to one of the insulated wires within the multi-conductor cable.
For electrical resistivity tomography, the data is described as the ratio of the induced potential over the current flow. The ratio of electrical potential over inducing current is referred to as the transfer resistance and has units of Ohms.
In addition to this data, electrical impedance tomography systems also measure time or frequency variations in the intrinsic electrical properties by measuring one of the following data quantities:
1) the time decay of the induced potential after the inducing current is turned off as either post-turnoff potential over pre-turnoff potential (in units of millivolts per volt) or the post-turnoff potential (in units of milliseconds integrated over a period of time and then divided by the pre-turnoff average potential);
2) the amplitude and phase of the induced potential for a sinusoidally varying current (in degrees or milliradians); or
3) percent frequency effect, which is the change in the transfer resistance of the received signal as the frequency of the inducing current is changed. This is sometimes presented as the change in transfer resistance over the transfer resistance at the lower frequency for measurements collected using a lower frequency one tenth that of the higher frequency.
The listing herein of these quantities does not necessarily imply any limitation on the abilities or goals of EIT systems, but is included merely to offer examples.
As described above, the electrodes are often disposed on a well casing provided within a borehole. The electrodes communicate with sensor equipment on the surface via a cable. To be effective, the electrode within the borehole must be coupled to both the cable and the native formation providing low electrical resistance while at the same time being strongly insulated from the highly conductive metal casing.
What is required are improved systems and methods for reducing the effects of well casing as well as improved systems and methods for connecting electrodes to the cable wires.